An endless hot rolling process has an advantage in that the manufacturing time may be significantly shortened, for example, by rolling a steel sheet within one second. Therefore, it is expected that the endless hot rolling process is used to increase productivity and improve product quality of steel sheet, as well as to prevent a variety of facility accidents, compared to the conventional rolling processes of standing by for a tens-second period from when one hot rolling material is rolled in a roughing mill to when the next one is rolled in a roughing mill. Also, the endless hot rolling process has advantages in that inferior front and rear ends of a coil and scrap loss may be reduced during the coiling process, and it is possible to produce a steel sheet having such width and thickness that it is difficult to roll a steel sheet in general hot rolling mill plants.
FIG. 1 is a configuration view illustrating a conventional endless hot rolling mill facility. Referring to FIG. 1, a hot rolling material rolled in a roughing mill 1 is coiled in a coil box 2, and a coil was cut between the rear end of a preceding coil material and the front end of a following coil material using a crop cutter 3, the preceding and following coil materials being drawn out from the coil box 2. Then, the drawing of the following coil material was accelerated so that the front end of the following coil material can be overlapped with the rear end of the preceding coil material, and the overlapped regions are joined with a joining machine 4. Then, the joined coil is finish-rolled in a finishing mill 5, and then cut with high-speed cutter 6 arranged in the front of the coiling machine 7, thus to produce a final product.
When the joint parts are cut with the high-speed cutter 6 of the endless hot rolling mill facility, it is important to secure exact tracking of joint parts and reduce errors in cutting length (a distance between a joint part and a cutting point), which leads to an increase in an actual yield of the final product. Therefore, there have been continuous demands for a technology of detecting junctions in an endless rolling material.
As the conventional methods of detecting junctions in an endless rolling material, there are known methods, such as a method including: producing a junction in an rolling material by joining overlapped region of the rolling material and tracking the corresponding junctions by using the rolling speed and the difference in thicknesses of a steel strip measured in an input and an output of the finishing mill (Korean Patent No. 0231980), a method for detecting a joint part including: detecting a joint part of a preceding coil material and a following coil material using the change in load of the rolling material or the change in a thickness direction (Korean Patent No. 0543258), etc.
In the case of these methods, a position of the joint part is determined by calculating a sheet pressure and a strand roll speed of a strand in the output of the finishing mill. Even when a joint part is exactly detected from a strand (for example, a first or second strand) arranged in an upper region of the joining machine 4 or the finishing mill 5 whose roll force is changed, a position of the joint part is determined by calculating a sheet pressure and a strand roll speed of the strand arranged in a lower region of the finishing mill 5 (for example, third to eighth stands). Therefore, errors in the position of the joint part are found from the final stand in the output of the finishing mill 5.
Also, there is a method of determining a joint part using an out-of-line sheet width and a crack length of the endless joint part. However, this method has its limit to determine a joint part since the sheet width may not be out of line and cracks may not be generated when coil materials having the same width are exactly overlapped and joined each other, or there is a sufficient joint strength in an edge of the joint part.